The present invention relates generally to circuit protection devices, and more particularly to a device that suppresses transient current/voltage surges.
Many of today""s highly sensitive electronic components, such as computer and computer-related equipment, that are used in commercial and residential applications contain transient voltage surge suppression (TVSS) devices. These devices protect sensitive and/or expensive electronic circuits and components from damage from over-voltage fault conditions. Such transient voltage surge suppression systems are typically designed for moderate fault conditions expected in normal use. In this respect, such systems are designed to suppress relatively minor fault conditions, but are not designed to protect against major over-voltage conditions. Examples of major over-voltage conditions include those that may occur from losing the system neutral or ground termination, or from repetitive current pulses as from lightning strikes. Such major over-voltage conditions can have catastrophic effects on sensitive electronic circuits and components. To prevent such fault conditions from reaching and damaging electronic circuits, components and equipment, it has been known to utilize larger voltage surge suppression devices. These devices are typically deployed at a building""s incoming electrical service power lines, or within a building""s power distribution grid to control power surges in the electrical lines to the building, or in the electrical lines to specific floors of the building. Such voltage surge suppression devices typically include a plurality of metal-oxide varistors (MOVs) connected in parallel between a service power line and a ground or neutral line, or between a neutral line and a ground line.
MOVs are non-linear, electronic devices made of ceramic-like materials comprising zinc-oxide grains and a complex amorphous inner granular material. Over a wide range of current, the voltage remains within a narrow band commonly called the varistor voltage. A log-log plot of the instantaneous voltage in volts versus the instantaneous current in amps yields a nearly horizontal line. It is this unique current-voltage characteristic that makes MOVs ideal devices for protection of sensitive electronic circuits against electrical surges, over-voltages, faults or shorts.
When exposed to voltages exceeding their voltage value, MOVs become highly conductive devices that absorb and dissipate the energy related to the overvoltage and simultaneously limit dump current to a neutral line or ground plane. If an over-voltage condition is not discontinued, the MOVs will continue to overheat and can ultimately fail catastrophically, i.e., rupture or explode. Such catastrophic failure may destroy the sensitive electronic equipment and components in the vicinity of the MOVs. The destruction of electrical equipment or components in the electrical distribution system can disrupt power to buildings or floors for prolonged periods of time until such components are replaced or repaired. Moreover, the failure of the MOVs in a surge suppression system may allow the fault condition to reach the sensitive electronic equipment the system was designed to protect.
In U.S. Pat. No. 6,040,971 to Martenson et al., entitled CIRCUIT PROTECTION DEVICE, there is disclosed a voltage suppression device for protecting an array of metal oxide varistors in a surge suppression system. The device was operable to drop offline an entire array of MOVs in the event that a voltage surge reached a level wherein one or more of the MOVs in the array might catastrophically fail. In the disclosed device and system, a trigger MOV was designed to have a lower voltage rating than any of the MOVs in the array. Thus, the entire array would drop offline in the event that a surge condition exceeded the voltage rating of the trigger MOV. In some instances, however, it may be desirable to maintain the array of MOVs active and to drop offline only those MOVs sensing a voltage surge exceeding the voltage rating of that particular MOV.
The present invention provides a circuit protection device, and a transient voltage surge suppression system incorporating such device, to protect an electrical system from catastrophic failure due to excessive over-voltage conditions or repetitive fault conditions.
In accordance with the present invention, there is provided a voltage suppression device for suppressing voltage surges in an electrical circuit. The device is comprised of a voltage sensitive element having a first surface and a second surface and a predetermined voltage rating across the first and second surfaces. The voltage sensitive element increases in temperature as the voltage applied across the first and second surfaces exceeds the voltage rating. A first terminal has one end electrically connected to the first surface of the voltage sensitive element and the other end of the terminal is connected to a ground or neutral line of an electrical circuit. A thermal element is electrically connected to the second surface of the voltage sensitive element, the thermal element being an electrically conductive solid at room temperature and having a predetermined softening temperature. A second terminal has one end in electrical connection with the second surface of the thermal element and another end connected to an electrical power line of an electrical circuit. The voltage sensitive element senses the voltage drop between the electrical power line and ground or neutral line. The second terminal is maintained in contact with the thermal element by the thermal element and is biased away therefrom. The second terminal moves away from electrical contact with the thermal element and breaks the electrical current path if an over-voltage condition sensed by the voltage sensitive element exceeds the voltage rating of the voltage sensitive element. Such an over-voltage causes the voltage sensitive element to heat the thermal element beyond its softening point. An arc shield moves from a first position wherein the arc shield allows contact between the second terminal and the thermal element to a second position wherein the shield is disposed between the second contact and the thermal element, i.e., when the second terminal moves from electrical contact with the thermal element.
In accordance with another aspect of the present invention, there is provided a voltage suppression device for suppressing voltage surges in an electrical circuit. The device is comprised of a voltage sensitive element having a predetermined voltage rating. The voltage sensitive element increases in temperature as voltage applied across the voltage sensitive element exceeds the voltage rating. Terminals electrically connect the voltage sensitive element between a power line of an electrical circuit and a ground or neutral line of the electrical circuit. A normally closed, thermal switch is electrically connected in series with the voltage sensitive element between the power line and the voltage sensitive element. The thermal switch is thermally coupled to the voltage sensitive element wherein the thermal switch moves from a normally closed position to an open position to form a gap between the thermal switch and the voltage sensitive element when the temperature of the voltage sensitive element reaches a level indicating an over-voltage condition. A non-conductive barrier is operable to move into the gap when the thermal switch moves to an open position. The barrier prevents line voltage surges from arcing between the thermal switch and the voltage sensitive element.
It is an object of the present invention to provide a circuit protection device to protect sensitive circuit components and systems from current and voltage surges.
It is another object of the present invention to provide a circuit protection device as described above to prevent catastrophic failure of a transient voltage surge suppression (TVSS) system within a circuit that may occur from repetitive circuit faults or from a single fault of excessive proportion.
A further object of the present invention is to provide a circuit protection device as described above that includes a current suppression device and a voltage suppression device.
Another object of the present invention is to provide a circuit protection device as described above for protecting a transient voltage surge suppression system having metal-oxide varistors (MOVs).
A still further object of the present invention is to provide a circuit protection device as described above that includes a metal-oxide varistor as a circuit-breaking device.
A still further object of the present invention is to provide a circuit protection device as described above that is modular in design and easily replaceable in a circuit.
These and other objects and advantages will become apparent from the following description of a preferred embodiment of the present invention taken together with the accompanying drawings.